Silicon Carbide is an exceptionally hard, corrosion and erosion resistant ceramic material. It has been recognized as a potential candidate material for many structural applications due to its unique combination of positive attributes such as, for example, excellent oxidation, corrosion and wear resistance as well as high hardness. Silicon Carbide is also known for its superior, high temperature mechanical properties such as strength retention and creep resistance.
Despite these positive attributes, the range of potential applications for Silicon Carbide material is severely limited due to deficiencies resulting from poor fracture toughness, inadequate thermal shock resistance and, accordingly, lack of total reliability.
Prior investigations have demonstrated that the fracture toughness of polycrystalline ceramics can be improved significantly through reinforcement with single crystal ceramic whiskers. For example, it has been reported that the addition of SiC whiskers not only improved the thermal shock resistance (.DELTA.T.sub.c) of Al.sub.2 O.sub.3 from 300.degree. to 1000.degree. C. but also improved its strength and reliability as measured by Weibull modulus calculations.
When whiskers are employed to reinforce ceramic materials, the reinforced regions dissipate energy during crack propagation. The efficiency of the reinforcement is dependent upon the physical and mechanical characteristics of the reinforcement, reinforcement loading and/or the nature of the reinforcement/matrix bonding. Matrix chemistry, reinforcement surface properties and processing parameters are primary considerations for the development of optimum interface conditions. It is important to understand that reinforcement will not prevent catastrophic failure of so reinforced ceramic materially, however, such reinforcement will effectively increase the stress threshold necessary to initiate a graceful failure.
In the prior art, it is known to reinforce ceramic materials through the use of whiskers which are pre-formed and are thereafter blended with the ceramic matrix powder during the manufacture of the ceramic material. Such processes have been difficult to carry out successfully because the tendency is for the whiskers to agglomerate, that is, for the whiskers to mix with the ceramic matrix powder non-uniformly, with some regions of the ceramic matrix powder having a high density of whiskers and other regions of the ceramic matrix powder having a low density of whiskers. Such non-uniformity of distribution of pre-formed whiskers within the ceramic matrix powder results in a corresponding non-uniformity in the final finished ceramic material with a concurrent lack of homogeneity of the final ceramic product and differing performance characteristics of the finished ceramic material at different regions thereof. Applicants have found that it is not feasible to obtain uniformity of dispersion of a desired amount of pre-formed whiskers within a ceramic matrix. Applicants have found that as the by weight of whiskers within the entire powder mix increases, clumping of whiskers increases. Within desirable ranges of % by weight pre-formed whisker addition, clumping is evident. Furthermore, quantities of pre-formed whiskers always include particulates mixed therein which, though made of the same material as the whiskers, materially affect the reinforcing ability thereof. This problem is an inherent result from the pre-formed whisker manufacturing process. Additionally, pre-formed whiskers as a raw material source are very expensive and usually cost more than $1,000/Kg.
In order to attempt to achieve more uniform homogeneity, that is, more uniform distribution of pre-formed whiskers within the ceramic material, it has been found that it is necessary to process the ceramic matrix powder/whisker composition using wet procedures concurrently with chemical treatments to achieve deflocculation and/or flocculation and to thereby preclude sedimentation.
In a further aspect, attempts have been made to employ methods such as turbo milling and high shear grinding to attempt to prevent whisker agglomeration and to attempt to uniformly distribute whiskers throughout the mixture. Turbo milling has been shown to be effective in homogenizing powder/whisker constituents and in improving the whisker characteristics. Other methods have been employed such as colloidal processing, chemical preparation of the powder and mixing in solution. Results are uncertain, at best. These methods fail to (1) effectively prevent agglomeration of whiskers and (2) uniformly distribute whiskers throughout the mixture.
In a further aspect, pre-formed whiskers are generally single crystals with a whisker diameter between 0.5 and 2.0 micrometers and an aspect ratio (length to diameter ratio) between 8 and 40. Whisker materials of this size and shape are known to be carcinogenic upon inhalation. Therefore, they represent a considerable health hazard and can be compared to asbestos-type materials in this regard. Whiskers must be handled with care, protective clothing and breathing devices are required. Growing whiskers in-situ, as accomplished in accordance with the teachings of the present invention, mitigates these problems.
Accordingly, a need has evolved to develop a method of reinforcement of ceramic materials through the use of whiskers, which method is reproducible, maintains homogeneity and uniform distribution of whiskers throughout the ceramic material is inexpensive and presents no health hazards, and, thus, provides the improvements in ceramic material performance for which the use of whisker reinforcements has shown great promise. This goal is an important aspect of the development of the present invention.
Ideally, the fabrication method should incorporate in-situ growth to preclude handling of pre-formed whiskers and use a material additive such as a particle as a precursor to in-situ growth. Powder particles are relatively inexpensive, easy to process and handle, making them more attractive as an additive than whiskers, platelets, etc.
With the decision made to develop a cost effective whisker reinforced ceramic material having an appropriate whisker population uniformly distributed throughout the ceramic matrix, it is important to develop an effective method of fabricating such a material. Due to the low diffusivity resulting from covalent bonding, SiC cannot be densified without the use of additives. As is known, solid state sintering of SiC is usually performed at temperatures above 2000.degree. C. As is also known, typical additives used in solid state sintering process include elemental Boron, Boron compounds, Aluminum compounds and Beryllium compounds. These additives are generally employed in combination with elemental Carbon. At the temperatures at which solid state sintering is conducted, Applicants have found that severe degradation of known whisker reinforcement materials will inevitably occur, thereby defeating the purpose for whisker reinforcement. In this regard, a particularly effective finished ceramic material consists of Silicon Carbide (SiC) having reinforcements consisting of Silicon Nitride (Si.sub.3 N.sub.4) whiskers. However, Applicants have found that if sintering temperatures exceed 1850.degree. C., the Silicon Nitride particles, precursors for reinforcing whiskers tend to be unstable. Furthermore, sintering temperatures below 1850.degree. C. have been found, by Applicants, to be favored to produce a fine, uniform microstructure having SiC grains averaging, at most, 1 micrometer. That is, exaggerated grain growth is generally not observed below 1850.degree. C. Such a microstructure is required in order to maintain good mechanical properties, to enhance reliability and to produce intergranular failure which deters fracture propagation and improves toughness. As such, a need has developed for a process for manufacture of whisker reinforced ceramic materials having a sintering step which takes plate at or below 1850.degree. C.
It is with these goals in mind that the present invention was developed.
The following prior art is known to Applicant:
In Volume 62, Number 7-8 of the Journal of American Ceramic Society, July-August, 1979, B. R. Lawn and D. B. Marshall proposed a ratio H/K.sub.IC as an index of "brittleness" where H is the hardness, or resistance to deformation and K.sub.IC is toughness, or resistance to fracture. In the article, the authors indicate that all materials are more susceptible to deformation in small-scale loading events and such materials are more susceptible to fracture in large-scale events. Such events are somewhat predictable based upon the hardness and toughness of the subject material. Thus, by normalizing the characteristic dimensions of the two competing processes and the contact load in terms of the appropriate functions of H and K.sub.IC, the authors have developed a universal deformation/fracture diagram. This diagram has since been employed to predict the mechanical response of any material of known hardness and toughness for any prospective in-service contact loading conditions. While this disclosure has nothing to do with the specific invention disclosed herein, the ratio H/K.sub.IC was employed by Applicants in determining the effectiveness of the finished ceramic materials disclosed herein.
In the Ceramic Bulletin, Volume 64, Number 2 (1985), George C. Wei and Paul F. Becher disclosed dense, toughened SiC whisker reinforced ceramic matrices. These materials were created using hot pressing procedures carried out on powders having pre-formed whiskers mixed therein. The authors describe hot pressing procedures conducted at temperatures ranging from 1250.degree. C. to 2000.degree.. The present invention differs from the teachings of Wei and Becher as contemplating growing of single crystal whiskers in-situ from particle precursors and of the use of liquid phase sintering techniques carried out at temperatures below 1850.degree. C.
U.S. Pat. No. 4,543,345 to George C. Wei and the corresponding Reissue Patent No. Re. 32,843 disclose ceramic composites, particularly Al.sub.2 O.sub.3, mullite or B.sub.4 C reinforced with pre-formed SiC whiskers. The present invention is distinct from the teachings of Wei in several respects. Firstly, Applicants herein disclose growing of single crystal whiskers in-situ from particle precursors. Secondly, Wei excludes SiC from consideration as a ceramic matrix material (column 3, lines 21-31). Furthermore, Wei acknowledges the inherent clumping of pre-formed whiskers which is eliminated when they are formed in-situ.
In a 1985 article by T. N. Tiegs and P. F. Becher titled "Whisker Reinforced Ceramic Composites", the authors disclose whisker reinforced ceramics wherein the whiskers are pre-formed and are intermixed and dispersed within the ceramic powder prior to high temperature processing. The authors describe hot pressing in a graphite dye at temperatures up to 1850.degree. C. The present invention differs from the teachings of Tiegs and Becher as contemplating growth of whiskers in a ceramic material in-situ from particle precursors along with liquid phase sintering at temperatures below 1850.degree. C.
In 1987, Messrs. Becher and Tiegs had an article published in the Engineered Materials Handbook, Volume 1, titled "Whisker-Reinforced Ceramics". This paper fails to teach insitu grown whiskers nor the use of liquid phase sintering at below 1850.degree. C. as disclosed herein.